This invention relates to a marine propulsion system, and more particularly to cooling of exhaust in an outboard marine propulsion system.
In an outboard marine propulsion system including a powerhead having a water cooled engine and a depending driveshaft housing and gearcase, water is generally taken in through intake passages formed in the submerged portion of the gearcase and supplied to a water pump, which circulates water to the engine cooling system. In a typical construction, an adaptor plate is positioned between and fastened to both the upper end of the driveshaft housing and the underside of the engine. The adaptor plate includes a passage for allowing cooling water to pass between the driveshaft housing and the engine cooling water inlet. Cooling water then circulates through the engine cooling system, and is routed back to the adaptor plate for discharge through a discharge opening formed therein and into the driveshaft housing.
The upper end of the driveshaft housing has an exhaust cavity formed therein, into which exhaust is discharged from the engine. The lower end of the exhaust cavity is provided with an exhaust outlet, through which exhaust passes for ultimate discharge through the hub of the propeller. According to current design, the upper wall of the exhaust cavity is defined by the adaptor plate. An opening is typically provided in the adaptor plate upstream of the cooling water discharge for diverting a portion of the engine cooling water into the exhaust cavity for cooling exhaust contained therein. However, such water supplied to the exhaust cavity has been circulated through the engine cooling system and heated thereby, thus reducing its ability to cool the exhaust. Further, such water is introduced into the exhaust cavity at the discharge end of the engine cooling system, where the cooling water is under relatively low pressure. Accordingly, a relatively small amount of water is provided to the exhaust cavity.
The adaptor plate typically includes a depending stub having an exhaust passage for passing exhaust from the engine exhaust discharge outlet into the driveshaft housing exhaust cavity. With this design, exhaust is introduced into the exhaust cavity at a point removed from the introduction of cooling water into the exhaust cavity. The exhaust passage formed in the depending stub of the adaptor plate is adapted to receive an exhaust pipe which extends downwardly into the exhaust cavity for providing induction tuning to the engine. With the provision of the exhaust pipe, exhaust is discharged therefrom into the lower portion of the exhaust cavity, which is typically submerged during boat operation. The induction tuning provided by the exhaust pipe increases the engine horsepower by increasing the volume of air taken into the engine cylinders, according to known induction tuning principles.
A problem resulting from the above-described construction is that, when the exhaust pipe is not in place, exhaust is discharged into the exhaust cavity at a point above the water line during boat operation. The high temperature engine exhaust thus excessively heats the upper walls of the driveshaft housing defining the exhaust cavity and located above the water line. The present invention is designed to alleviate this problem.
In accordance with the invention, the outboard marine propulsion system as described is provided with exhaust cooling means for providing cooling water to exhaust flowing through the engine exhaust discharge outlet prior to its entrance into the exhaust cavity. The cooling water is preferably introduced to the exhaust prior to circulation of the cooling water through the cooling system of the engine. The cooling water is thus supplied to the exhaust prior to being heated by the engine, and while under relatively high pressure due to its proximity to the water pump. The cooling water is preferably provided from a point adjacent the engine cooling water inlet.
In one embodiment of the engine, the engine cooling water inlet and the engine exhaust discharge outlet are separated by a wall, and a passage is formed in the wall for diverting a portion of the cooling water from the engine cooling water inlet to the engine exhaust discharge outlet for cooling the exhaust prior to its entrance into the driveshaft housing exhaust cavity.
In another embodiment of the invention, the adaptor plate includes a passage extending between the adaptor plate cooling water passage and the adaptor plate exhaust discharge outlet, for diverting a portion of the cooling water from the engine cooling water inlet to cool the exhaust prior to its discharge into the exhaust cavity. Two adaptor plate structures are disclosed for carrying out this embodiment of the invention. In a first structure, the adaptor plate is provided with an upwardly facing trough formed in its upper surface. The trough has an inlet in communication with the engine cooling water inlet, and an outlet in communication with the adaptor plate exhaust passage. In a second structure, the passage inlet is in communication with the engine cooling water inlet, and the passage outlet is disposed at a point downstream of the inlet to the adaptor plate exhaust passage. According to the latter structure, positioning of the induction tuning exhaust pipe into the adaptor plate exhaust passage plugs the cooling water passage outlet for blocking the flow of cooling water therethrough.
The invention further contemplates an adaptor plate constructed as detailed above. Also disclosed is a method of cooling exhaust in an outboard system, substantially according to the foregoing summary.